When Santa Fe author and archaeoastronomer Anna Sofaer discovered the Sun Dagger near the top of Fajada Butte at Chaco Canyon in 1977, she tumbled headlong into an enduring mystery that has led to scientific papers, projects, books, academic controversies, documentaries and interactive computer models. Sofaer will lead a presentation about her work and findings from 7-8:30 p.m. Thursday at the Bradbury Museum.All places have their histories, but New Mexico is fortunate in the United States to have an extremely rich prehistory, much of it centered and symbolized in what was once a major urban outpost in the San Juan basin in the northwestern corner of the state. However dimly Chaco culture is perceived, now 1,000 years and more since it flourished, the importance and significance of the ancient architecture and the vast ritual landscape at Chaco Canyon continue to dazzle tourists and puzzle scholars.Anna Sofaer’s Solstice Project has produced two documentaries, both narrated by Robert Redford. The first, which she produced, directed and wrote in 1982, told the story of “The Sun Dagger,” the astronomical calendar at Chaco and the profound revelation she and her team deduced about its relation to the movements of the sun and moon.She summarized the basic functions of the device in a paper two years ago: “(T)hree upright sandstone slabs cast precise light and shadow patterns on two spiral petroglyphs, recording the summer and winter solstices, the equinoxes and the 18.6 year lunar cycle.”Summer solstice is the longest day of the year in the Northern Hemisphere, marking the limit of the sun at its northernmost point, when days begin to grow shorter again. At Chaco, moments before noon on the summer solstice, Sofaer found a thin dagger of light between the two of the slabs fell directly through the middle of the spiral.On winter solstice, the shortest day, two vertical and parallel daggers of light are cast just before solar noon that fit exactly along the sides of the larger spiral that has been pecked into a facing rock.A second film, “The Mystery of Chaco Canyon” (2000), explored the many striking astronomical features and solar-lunar alignments of the center’s buildings and roads, suggesting an even deeper and more elaborate cosmological significance for the site. The substance of these themes and discoveries are pulled together in the new book, “Chaco Astronomy – An Ancient American Cosmology,” by Sofaer and the other contributors to the Solstice Project, including Native Americans, archeologists and astronomers. This compilation of key papers was just published by Ocean Tree Books of Santa Fe.Because of Sofaer’s vigilance and close scrutiny of the Sun Dagger, she noticed that the light markings changed between 1979 and 1989, due to shifts in the stone slabs that now alter the image cast from a sharp dagger straight through the heart of the spiral. The light now appears as an irregular carrot-shaped object slightly off center.Using photographic and photogrammetric records, the Solstice team was able to prove the dimensions and causes of the change and in the process they began building a laser-scanned model of the Sun Dagger that has now become an important educational and archival tool in its own right.Ohio State University’s Allan Price, who first worked on computer animation for the Chaco documentary, developed the application for interacting with the Sun Dagger model. He will demonstrate the software during Thursday’s event and explain how it can be used by someone familiar with the site.One researcher who has worked with the Sun Dagger model is Ben Luce, a former physicist Los Alamos National Laboratory. Luce has recently moved to Vermont, where he is writing a book.His project with the Sun Dagger model has tried to use mathematical models to understand the phenomenon better.“The conclusion is that the Sun Dagger is what we would call a uniquely determined construction,” he said in a telephone interview. “That means it is not over-determined. That is, it would have taken a miracle in the god-given shapes of the rocks to achieve the effects.”He added, “Nor is it underdetermined, meaning that any old position would have given you the alignments. This also suggests it was not an accident.”How did they do it then?“I tend to think it was conceptual in the beginning,” Sofaer said in a recent interview.Her conclusions have now been built up meticulously through three decades and have won over many of her colleagues, who have found supportive lines of evidence for a subject that nevertheless remains controversial.Sofaer’s visit to Los Alamos offers a rare access not only to a fascinating body of research but also a glimpse into the unwritten book of our continent and the wisdom of its people.

Sofaer will sign copies of her book Thursday as well; see story on page B1 of the Feb. 17 print edition.